Theresa May has been rebuked by the UK's official statistician over her criticism of the performance of the NHS in Wales.
Mrs May went on the attack during Prime Minister's Questions in the House of Commons last month with figures apparently showing that more than seven times as many patients were waiting over 12 hours in Welsh casualty departments than in England.
But the chair of the UK Statistics Authority Sir David Norgrove has now said the comparison used by the Prime Minister was "not valid".
In a letter to Welsh First Minister Carwyn Jones, who complained about the PM's comments, Sir David said: "You are right to say that the comparison is not valid.
"The figure used for England refers to the accident and emergency wait time from the decision to admit to admission into another part of the health service.
"The figure used for Wales represents the entire time patients wait
from arriving to leaving accident and emergency services, including the time from decision to admit to actual admission."
Ministers including Mrs May have repeatedly highlighted what they see as the poor performance of the NHS in Labour-run Wales as they sought to defend their stewardship of the health service in England.
On January 25, Mrs May was responding to questions from Labour leader Jeremy Corbyn about a winter crisis which he said had seen NHS England record its worst-ever A&E waiting time figures.
She said: "If he wants to talk about figures and about targets being missed, yes, the latest figures show that, in England, 497 people were waiting more than 12 hours, but the latest figures also show that, under the Labour Government in Wales, 3,741 people were waiting more than 12 hours."
Mr Jones wrote to the Statistics Authority to complain of the PM's "misleading" comments, and warning: "Selective misuse of statistics like this does not allow for a fair debate on the NHS.
"Research such as the OECD Review of Health Care Quality showed that there was no consistent picture of one nation's health system performing better than another."